A transient uterine inflammation post-breeding is a normal physiological reaction in the mare, and it is believed that the inflammatory response is necessary to eliminate bacteria and excess spermatozoa introduced into the uterine lumen. A tight balance between multiple pro- and anti-inflammatory factors is required for resolving the breeding-induced inflammation within 24–36 hr in the reproductively healthy mare, whereas a subpopulation of mares is susceptible to development of a persistent infection that can interfere with fertility. The aetiology of persistent endometritis can be either bacterial or semen-induced and both scenarios can threaten the establishment of pregnancy. Several factors associated with susceptibility to persistent endometritis have been identified including altered innate immune response in the early inflammatory process, reduced myometrial contractions and impaired opsonization; however, the pathogenesis to susceptibility has not been fully elucidated. Current research focuses on the initial hours of uterine inflammatory responses to semen and bacteria, and potential treatments to modify this altered innate immune response. An increased understanding of the mechanisms involved in the disease progression is necessary to improve the treatment and management of these mares. This review attempts to summarize the current knowledge of the uterine inflammatory and immunological responses to breeding-induced endometritis, persistent breeding-induced endometritis (PBIE) and bacterial endometritis in the mare.